The beaks of the finches
This week's paper is "Divergence with gene ﬂow as facilitated by ecological differences: within-island variation in Darwin's ﬁnches", published in Philosophical Transactions of the Royal Society.
"If humans evolved from chimps", ask the creationists, "why are there still chimps?" Good question! Humans didn't evolve from modern chimps, any more than I am descended from my cousins. We just share ancestors.
But still... why are there now chimps and humans, rather than one species? More generally, how does one species split in two? If part of a species becomes isolated enough to be inaccessible for mating, or at least "geographically undesirable," then the two populations can diverge through natural selection or random genetic drift, eventually losing the ability to reproduce with each other.
But genetic divergence doesn't always require geographical isolation. For example, if birds with medium-size beaks get less food than those with large or small beaks (on an island with many large and small seeds but few medium seeds), will the resulting "disruptive selection" tend to split the population into two subpopulations, with genes for small and large beaks? Maybe, but not if small- and large-beaked birds interbreed, combining genes for large and small beaks. And why wouldn't they?
The Galapagos Islands host several finch species, which vary greatly in beak size and shape. But what about differences in beak size within a species? Usually, that shows some form of normal ("bell-curve") distribution. Peter and Rosemary Grant have studied these finches for decades, showing how wet and dry years favor one end or the other of this distribution.
This week's paper discusses Geospiza fortis finches from one location on Santa Cruz Island. This population is unusual because it has a bimodal (two-peak) distribution in beak size. Genetic differences between smaller- and larger-beaked finches were statistically significant and greater than genetic differences among locations on the same island. Based on these data, they calculated gene flow (essentially, interbreeding) and concluded that smaller- and larger-beaked birds mated less with each other, especially at the site with bimodal beak size, than they did with finches with similar beak size from other parts of the island.
Why wouldn't a smaller-beaked bird mate with a larger-beaked one, or vice versa? Sure, they eat different sizes of seeds, but so what? Female finches prefer to mate with males whose songs are like the ones their fathers used to sing. And previous work by the authors shows that beak size has a major effect on songs. So females tend to mate with males with similar beak size, limiting the gene flow or interbreeding that would prevent divergence in bill size.
But a low rate of interbreeding wouldn't cause divergence in bill size. That would probably require disruptive selection, in which finches with intermediate bill sizes have lower survival or reproduction. Previous work by the authors did seem to show lower survival for birds with intermediate-size beaks. Finches with intermediate bill size were more likely to disappear from one year to the next. But did they die? Or did they fly to the other end of the island, where potential mates weren't quite so fussy?
Either way, this paper and others by the authors are an impressive demonstration of how much we still can learn, even from such highly-studied species as Darwin's finches.